1. Field of the Invention
The present invention generally relates to liquid-crystal-display devices, and particularly relates to correction of view-angle-dependent characteristics of a liquid-crystal-display device.
2. Description of the Related Art
An LCD (liquid crystal display) has view-angle-dependent characteristics such that brightness of a pixel varies, depending on from which angle the pixel is viewed, even when a voltage applied to the pixel is kept constant. Because of this, an attempt to display the same brightness level over the whole display by applying the same voltage to all the pixels fails to accomplish an intended result. That is, the display shows different brightness at a different portion thereof since a view angle of an eye position relative to each pixel differs from pixel to pixel. Where full-color display requiring half-tone representation is employed, the view-angle-dependent characteristics are regarded as a main factor contributing to degradation of image quality when dark images are displayed.
FIGS. 1A through 1D are illustrative drawings showing T-V characteristics which depend on a view angle and relative positions between a liquid-crystal panel and an eye position. In FIGS. 1A and 1B, a vertical view angle with respect to a line L1 is shown as .theta.u when an eye position VP provides a view over a liquid-crystal panel 500 having pixel lines L1, L2, . . . , and Ln, and a vertical view angle is represented as .theta.d with respect to the line Ln at the bottom of the panel. The line Lm is situated at such a position that a view line extending from the eye position VP reaches this position at a right angle relative to the surface of the liquid-crystal panel 500. In FIGS. 1C and 1D, curves C1, Cm, and Cn show relations between a transparency ratio T and a applied voltage V when the line L1, Lm, and Ln, respectively, are viewed from the eye position VP.
As is apparent from FIG. 1C, the transparency ratio T when a pixel voltage Vb1 is applied differs with respect to each of the lines L1, Lm, and Ln. When the pixel voltage is changed to Vb2, an order of the brightness level is reversed between the line L1 and the line Lm, thereby making it hard to see the display. In order to achieve the same level t of the transparency ratio T, for example, different voltages should be applied as shown by Vc1, Vcm, and Vcn in FIG. 1D.
An attempt to improve view-angle-dependent characteristics of a liquid-crystal display has provided various schemes to date. An example of a mechanical scheme is to provide an adjustment mechanism for controlling an angle at which the liquid-crystal-display panel is placed, thereby allowing a user to set an appropriate angle. An example of an electrical scheme is to allow an adjustment to be made with regard to voltage levels applied to pixels such that a different range of the T-V characteristics (relations between the transparency ratio T and the applied voltage V) can be used. Related-art adjustment schemes, however, have various problems. For example, brighter portions may saturate when an attempt is made to improve image quality of darker portions. Also, appropriate correction cannot be made concurrently to the entirety of the display. Further, a circuit size tends to become undesirably bigger.
Accordingly, there is a need for a liquid-crystal display which can provide a high-quality image by using a simple circuit structure.